How many generations until a new species is formed?

For selective breeding in mammalian sexual reproduction, about how many generations would need to pass before a new species came into being, one that could no longer successfully breed with the starting stock?

I am unfamiliar with selection pressure. In a laboratory setting it would be possible to select for individuals with a particular trait, putting selection pressure at its maximum. Being prevented by an external agent from reproducing isnīt what counts in determining whether a new species has formed, though.

If selection pressure is at its maximum, how many generations would it take, I wonder. I donīt think it is known.

To give a number, sometimes even one generation may be enough, i.e. one mutation that then via selection or building a reproductive barrier separates the populations. But this is a very, very rare event, as the mutation has to spread and not eradicated or just outnumbered in a large population. Here is an example, with an approximately way how it could work.

One must presume that long and short arguments contribute to the same end. - Epicurus...except casandra's that did belong to the funniest, most interesting and imaginative (or over-imaginative?) ones, I suppose.